The broad, long-term objectives of this research are to elucidate the fundamental principles and mechanisms of hydrogen transfer in enzyme catalysis and to address unresolved issues in biologically important systems. These objectives will be accomplished with computational methods that include electronic and nuclear quantum effects, as well as the motion of the entire solvated enzyme. The calculations will probe the roles of electrostatics, hydrogen bonding, hydrogen tunneling, and protein motion in enzyme reactions. The four enzyme reactions that will be studied have been chosen on the basis of their biomedical importance and the availability of relevant experimental data. The first specific aim centers on the enzyme dihydrofolate reductase (DHFR), which is required for normal folate metabolism in prokaryotes and eukaryotes. This enzyme maintains tetrahydrofolate levels required to support the biosynthesis of purines, pyrimidines, and amino acids. DHFR is medically relevant in that inhibition of DHFR with potent antifolates has been used successfully in cancer chemotherapy. The second specific aim centers on the enzyme dihydroorotate dehydrogenase (DHOD). This enzyme catalyzes the only redox reaction in the biosynthesis of pyrimidines, which are required for the supply of precursors for RNA and DMA synthesis. DHOD is medically relevant in that the immunosuppressive effects of inhibiting this enzyme have been used therapeutically to treat diseases such as rheumatoid arthritis. The third specific aim centers on the enzyme lipoxygenase. This enzyme aids in the production of leukotrienes and lipoxins, which regulate responses in inflammation and immunity. In mammals, lipoxygenases are medically relevant in that inhibitors have been used as drug agents to treat diseases such as asthma, atherosclerosis, psoriasis, and cancer. The fourth specific aim centers on the enzyme ketosteroid isomerase (KSI), which catalyzes the isomerization of steroids. In mammals, this enzyme is medically relevant in that it controls the synthesis of steroid hormones. Deficiencies of KSI and related enzymes in humans lead to a wide range of diseases and health problems. All of these studies are relevant to public health because the elucidation of the mechanisms will facilitate the development of more effective drugs for a broad range of diseases, including cancer, asthma, malaria, and rheumatoid arthritis.